Best of them, and possibly the best API demo I’ve seen, is a text adventure run over telnet. Entering a UK postcode will select an MP for you, and you have to guide that MP to Tony Blair’s Sedgefield constituency, doing battle with any opposing MPs you pass along the way, and eventually fight the PM himself.
Entering my parents’ Tunbridge Wells postcode unfortunately selects me a Conservative MP, but in the spirit of role-playing I guided him to Sedgefield and defeated Mr. Blair. Greg Clark MP‘s verbal diarrhoea skills are not to be trifled with, and he picked up 3246 Experience Points along the way. Very entertaining.